1. Field of the Invention
The present invention relates generally to a fluid-filled elastic mount for damping or isolating vibrations based on flows of a non-compressible fluid contained therein. More particularly, the present invention is concerned with such a fluid-filled elastic mount suitably used as an engine mount of a motor vehicle, for example, which is provided with means for changing the vibration damping or isolating characteristics.
2. Discussion of the Prior Art
An elastic mount is known as a vibration damping member interposed between two members of a vibration system, for flexibly connecting these two members. This type of elastic mount usually includes a first and a second support structure which are spaced apart from each other in a load-receiving direction and are elastically connected to each other by an elastic body interposed therebetween. The elastic mount of this type is widely used for motor vehicles, for example, as a mount for mounting the engine on the vehicle body.
Recently, there is proposed a so-called fluid-filled elastic mount, in an effort to meet increasing requirements for improved vibration damping or isolating capability on the motor vehicles. An example of such a fluid-filled elastic mount is disclosed in laid-open Publication No. 55-107142 of unexamined Japanese Patent Application. The elastic mount disclosed in this publication includes a partition member supported by the second support structure so as to extend in the pressure-receiving chamber in a direction substantially perpendicular to the load-receiving direction, and thereby cooperate with the elastic body to at least partially define a pressure-receiving chamber on the side of the first support structure. The elastic mount also includes a flexible diaphragm which cooperates with the partition wall and the second support structure to define a variable-volume equilibrium chamber on the side of the second support structure. The pressure-receiving and equilibrium chambers are filled with a non-compressible fluid, and communicate with each other by an orifice passage. A pressure in the pressure-receiving chamber changes due to an elastic deformation of the elastic body upon application of vibrations in the load-receiving direction. This fluid-filled elastic mount isolates the input vibrations based on the resonance of the fluid mass flowing through the orifice passage, more effectively than the elastic mount which relies on only the elastic property of the elastic body for damping the input vibrations.
The required vibration damping or isolating characteristic of the fluid-filled elastic mount of the type discussed above differs depending upon the frequency of the vibrations applied. Therefore, the fluid-filled elastic mount must exhibit different vibration damping or isolating characteristics with respect to different frequency bands of the input vibrations. For example, the fluid-filled elastic mount when used as a vehicle engine mount is required to exhibit a high degree of damping characteristic with respect to low-frequency vibrations having frequencies in the neighborhood of 10-30 Hz, such as engine shake and bounce, and exhibit a sufficiently low dynamic spring constant with respect to medium- to high-frequency vibrations having frequencies of about 100-200 Hz, such as secondary vibrations of the engine and booming noise, while the vehicle is running. While the vehicle is at a stop with the engine idling, the fluid-filled elastic mount is required to exhibit a sufficiently low dynamic spring constant with respect to low-frequency vibrations having frequencies of about 15-30 Hz such as engine idling vibrations.
However, an improvement in the vibration isolating function based on the resonance of the fluid mass flowing through the orifice passage is provided with respect to only the vibrations whose frequencies are in the neighborhood of the frequency to which the orifice passage is tuned. The orifice passage is usually tuned by suitably determining its length and cross sectional area. Once the orifice passage is structurally tuned, the vibration damping or isolating characteristics of the elastic mount are fixed and are not adjustable. For instance, the orifice passage is tuned to effectively damp engine shake and bounce and other low-frequency vibrations based on the resonance of the fluid mass flowing through the orifice passage. In this case, the damping or isolating function of the elastic mount is considerably deteriorated with respect to the secondary vibrations of the engine, booming noises and other medium-to high-frequency vibrations, and with respect to the engine idling vibrations. Thus, the known fluid-filled elastic mount is not capable of effectively dealing with the various types of input vibrations having different frequencies.
In view of the above drawback of the known fluid-filled elastic mount, a fluid-filled elastic mount is proposed as disclosed in laid-open Publication Nos. 60-104824 and 60-249749, which has a resonance member supported by the first support structure such that a head portion of the resonance member extends in the pressure-receiving chamber in a direction substantially perpendicular to the load-receiving direction. The head portion of the resonance member and a surface defining the pressure-receiving chamber cooperate with each other to define therebetween a restricted portion, through which the fluid flows upon application of vibrations. Based on the resonance of the fluid flowing through this restricted portion of the pressure-receiving chamber, the proposed fluid-filled elastic mount exhibits a sufficiently low dynamic spring constant with respect to the medium- to high-frequency vibrations, which cause the orifice passage to operate as if the passage was substantially closed.
However, the fluid-filled elastic mount provided with the resonance member as discussed above still suffers from a drawback. Namely, this elastic mount is almost incapable of selectively exhibiting two different vibration damping or isolating characteristics, a high vibration damping effect with respect to the engine shake and bounce, and a low dynamic spring constant with respect to the engine idling vibrations, since these two types of vibrations have substantially the same frequency range. In this respect, there has been a long-felt need to improve the known fluid-filled elastic mount which utilizes a resonance member as described above.